Along with recent year's expansion of electronic technologies and growing concern in environmental technologies, various types of electrochemical devices have been used. Particularly, there are many demands for energy saving, and expectations for things capable of contributing to the demands increasingly become high. The things include, for example, solar batteries as power generation devices, and secondary batteries and capacitors as power storage devices. Lithium ion secondary batteries as a representative example of power storage devices have been originally used mainly as rechargeable batteries for portable devices, but in recent years, they have been expected to be used as batteries for hybrid cars and electric cars.
Now then, these electrochemical devices are used over a long period of several years to several tens of years, and demanded to exhibit a high efficiency and a low cost, and additionally, an elongated life (high durability) and high safety as well. However, since materials constituting these electrochemical devices include also materials easily combustible and materials easily deteriorating, there remains a problem in achievement of elongated life and high safety.
Lithium ion secondary batteries representative as power storage devices usually have a constitution in which a positive electrode and a negative electrode constituted mainly of active substances capable of doping and dedoping lithium are disposed across a separator. In the lithium ion secondary batteries, the positive electrode is formed by coating a positive electrode mixture obtained by mixing LiCoO2, LiNiO2, LiMn2O4 and the like as positive electrode active substances, carbon black, graphite and the like as conductive agents, and polyvinylidene fluoride, a latex, rubber and the like as binders, on a positive electrode current collector composed of aluminum or the like. The negative electrode is formed by coating a negative electrode mixture obtained by mixing coke, graphite and the like as negative electrode active substances, and polyvinylidene fluoride, a latex, rubber and the like as binders, on a negative electrode current collector composed of copper or the like. The separator is formed of a porous polyolefin or the like, and has a thickness as very thin as several micrometers to several hundred micrometers. The positive electrode, the negative electrode and the separator are immersed in an electrolyte solution in the battery. Examples of the electrolyte solution include electrolyte solutions in which a lithium salt such as LiPF6 and LiBF4 is dissolved in an aprotic solvent such as propylene carbonate and ethylene carbonate, or a polymer such as polyethylene oxide.
Lithium ion secondary batteries are presently used mainly as rechargeable batteries of portable devices (see, for example, Patent Literature 1). However, an organic solvent-based electrolyte solution is used in the lithium ion secondary batteries, so there arises a large problem of further improvement of the safety. For the improvement of the safety, batteries have been developed including batteries using an ionic liquid (see, for example, Patent Literatures 2 and 3) as an electrolyte solution, a polymer battery and a gel battery (see, for example, Patent Literatures 4 and 5), a battery in which an additive for improving safety is added to an electrolyte solution (Patent Literature 6), and a battery using a fluorosolvent as an electrolyte solution (see, for example, Non Patent Literature 1).